Method for determining an average cell voltage for fuel cells

ABSTRACT

A system and method for determining a maximum average cell voltage set-point for fuel cells in a fuel cell stack that considers oxidation of the catalyst in the fuel cells. The method includes determining the average cell voltage, the stack current density (I) and an internal resistance (R) of membranes in the fuel cells to calculate an IR corrected average cell voltage. The IR corrected average cell voltage is then used to determine the oxidation state of the catalyst particles using, for example, an empirical model. The oxidation state of the particles is then used to calculate the maximum average cell voltage set-point of the fuel cells, which is used to set the minimum power requested from the fuel cell stack.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Utility applicationSer. No. 13/107,526, now U.S. Pat. No. 8,647,785 titled Dynamic VoltageSuppression in a Fuel Cell System, filed May 13, 2011, which claims thebenefit of the filing date of U.S. Provisional Application Ser. No.61/382,724, titled Dynamic Voltage Suppression in a Fuel Cell System,filed Sep. 14, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and method for determininga maximum cell voltage for fuel cells in a fuel cell stack and, moreparticularly, to a system and method for determining a maximum cellvoltage for fuel cells in a fuel cell stack that includes determiningthe oxidation state of the fuel cell catalyst so that the maximum stackvoltage set-point can be adjusted during operation of the fuel cellsystem to minimize platinum catalyst surface area loss.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electro-chemical device that includes an anode and a cathode withan electrolyte there between. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated at theanode catalyst to generate free protons and electrons. The protons passthrough the electrolyte to the cathode. The protons react with theoxygen and the electrons at the cathode catalyst to generate water. Theelectrons from the anode cannot pass through the electrolyte, and thusare directed through a load to perform work before being sent to thecathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. A PEMFC generally includes a solid polymer electrolyte protonconducting membrane, such as a perfluorosulfonic acid membrane. Theanode and cathode typically, but not always, include finely dividedcatalytic particles, usually a highly active catalyst such as platinum(Pt) that is typically supported on carbon particles and mixed with anionomer. The catalytic mixture is deposited on opposing sides of themembrane. The combination of the anode catalytic mixture, the cathodecatalytic mixture and the membrane define a membrane electrode assembly(MEA). MEAs are relatively expensive to manufacture and require certainconditions for effective operation

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For example, a typical fuel cell stack for avehicle may have two hundred or more stacked fuel cells. The fuel cellstack receives a cathode reactant input gas, typically a flow of airforced through the stack by a compressor. Not all of the oxygen isconsumed by the stack and some of the air is output as a cathode exhaustgas that may include water as a stack by-product. The fuel cell stackalso receives an anode hydrogen reactant input gas that flows into theanode side of the stack.

A fuel cell stack typically includes a series of bipolar platespositioned between the several MEAs in the stack, where the bipolarplates and the MEAs are positioned between two end plates. The bipolarplates include an anode side and a cathode side for adjacent fuel cellsin the stack. Anode gas flow fields are provided on the anode side ofthe bipolar plates that allow the anode reactant gas to flow to therespective MEA. Cathode gas flow fields are provided on the cathode sideof the bipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack. The bipolar plates alsoinclude flow channels through which a cooling fluid flows.

It is known that a typical fuel cell stack will have a voltage loss ordegradation over the lifetime of the stack. It is believed that the fuelcell stack degradation is, among other things, a result of voltagecycling of the fuel cells in the stack. Voltage cycling occurs when theplatinum catalyst particles used to enhance the electro-chemicalreaction transition between a low and high potential state, whichpromotes dissolution of the particles. Dissolution of the particlesresults in loss of active surface area and performance degradation.

Many factors influence the relative loss in surface area of the platinumparticles relating to voltage cycling, including peak stack voltage,temperature, stack humidification, voltage cycling dynamics, etc. Lowerstack voltage set-points offer greater protection against degradation,but higher stack voltage set-points provide increased system efficiency.Thus, the control for various fuel cell systems often requires the stackto at least operate at a minimum power level so that, in at least onecase, the cell voltages are prevented from rising too high becausefrequent voltage cycles to high voltage can cause a reduction in theactive platinum surface area of the cathode and anode electrodes, asdiscussed above.

Typically, in known fuel cell systems, a fixed voltage limit is used toset the stack minimum power level to prevent unwanted voltage cycling.For example, a typical voltage suppression strategy may use a fixedvoltage set-point, such as 850-900 mV, and prevent the stack voltagefrom rising above that value. If the fuel cell power controller is notrequesting power, or is requesting minimal power, the power generated bythe stack necessary to maintain the cell voltage levels at or below thefixed voltage set-point is provided to certain sources where the poweris used or dissipated. For example, the excess power may be used tocharge a high voltage battery in a fuel cell system vehicle. U.S. PatentApplication Publication No. US 2006/014770 A1, published Jul. 6, 2006,titled Reduction of Voltage Loss Caused by Voltage Cycling by Use of ARechargeable Electric Storage Device, assigned to the assignee of thisapplication and herein incorporated by reference, discloses a fuel cellsystem that charges a vehicle battery in order to maintain the cellvoltage below a predetermined fixed voltage set-point.

If the voltage set-point is relatively high, then the system may oftencharge the battery, which could cause the battery charge to be at itsmaximum more often. If the battery is at its maximum charge and cannotexcept more charging power, then the controller may cause the excesspower to be dissipated in other components, such as resistors, in theform of heat to maintain the cell voltage below the maximum voltageset-point, which effects system efficiency as a result of wastinghydrogen fuel.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system andmethod are disclosed for determining a maximum average cell voltageset-point for fuel cells in a fuel cell stack that considers oxidationof the catalyst in the fuel cells. The method includes determining theaverage cell voltage, the stack current density (I) and an internalresistance (R) of membranes in the fuel cells to calculate an IRcorrected average cell voltage. The IR corrected average cell voltage isthen used to determine the oxidation state of the catalyst particlesusing, for example, an empirical model. The oxidation state of theparticles is then used to calculate the maximum average cell voltageset-point of the fuel cells, which is used to set the minimum powerrequested from the fuel cell stack.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a fuel cell system;

FIG. 2 is a graph with time on the horizontal axis, average cell voltageon the left vertical axis and minimum power request provided by a fuelcell power system on the right vertical axis; and

FIG. 3 is a flow chart diagram for determining and using a minimumrequested power value from a fuel cell stack that considers a cellcatalyst oxidation state during operation of a fuel cell stack.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa system and method for determining a maximum cell voltage set-point forfuel cells in a fuel cell stack that includes determining the oxidationstate of the fuel cell catalyst to minimize platinum catalyst surfacearea loss is merely exemplary in nature, and is in no way intended tolimit the invention or its applications or uses. For example, the systemand method of the present invention has particular application forestimating the voltage set-point of the fuel cells in a fuel cell stackfor a fuel cell vehicle. However, as would be appreciated by thoseskilled in the art, the system and method for estimating the peak stackvoltage will have application for other fuel cell stacks and otherapplications.

Oxidation of platinum particles in a fuel cell as a result of voltagecycling creates a passivation layer in the cell electrode that preventsthe particles from going into solution and being absorbed into themembrane. In other words, oxidation of the platinum particles in a fuelcell reduces the possibility of a reduction in catalyst surface area,which reduces cell degradation. Although the discussion herein refers tothe catalyst as being platinum, those skilled in the art will readilyunderstand that other metals can be used as a catalyst and that thecatalyst may be in various concentrations, particle sizes, supportmaterial, etc.

It is believed that the loss of platinum catalyst in a fuel cell stackMEA occurs as a result of two competing reactions occurring in the MEAas identified by equations (1) and (2) below.Pt+H₂O→PtOH+H⁺ +e−  (1)Pt→Pt²⁺+2e−  (2)

The reaction of equation (2) is believed to be damaging to the catalyst,but the reaction of equation (1) is believed to be protective of thecatalyst. Both of the reactions occur at high fuel cell voltagepotentials, such as voltages greater than 0.7V and especially cellvoltages greater than 0.9V. The reaction of equation (1) starts at alower potential and proceeds much slower than the reaction of equation(2), which proceeds very fast at high voltage potentials, i.e.,potentials greater than 0.85V.

The present invention proposes a process that reduces or prevents thereaction of equation (2), but favors the reaction of equation (1). Analgorithm is proposed that controls the cell voltage potential bymonitoring the PtOH levels using models, and keeps the potential low,i.e., less than 0.85V by capping the potential and/or rate of change ofthe potential until the PtOH level is high enough so that the reactionof equation (2) is prevented. Once the PtOH level is high, the cellvoltage can be allowed to increase without extensively damaging thecatalyst.

The algorithm determines a maximum average cell voltage set-point atvarious times during operation of the fuel cell system based on anestimate of the history of the cell voltage, the oxidation level andrate of oxidation of the platinum particles and the cell voltage. Moreparticularly, the voltage set-point of the cells may be caused to rampup from some lower voltage value to a relatively higher voltage value atsome predetermined rate depending on the oxidation level of the platinumparticles for that point in time. Therefore, as the power demand on thestack is reduced and the cell voltage increases, the amount of stackpower used to charge the battery, or be dissipated in some other device,may be reduced as the platinum particles are allowed to oxidize, as thecell voltage increases to some maximum voltage level set-point.

As will be discussed in detail below, the present invention includes amethod for periodically estimating the peak stack voltage of a fuel cellstack in a fuel cell system during operation of the fuel cell systemthat includes determining the platinum oxidation state. This estimatedpeak stack voltage allows the stack voltage set-point to be selectivelylow enough to provide protection against platinum catalyst surface arealoss, and be high enough to provide stack operating efficiency.Generally, given certain stack conditions at a certain point in time, analgorithm estimates a target maximum average cell voltage (MAV), andusing the MAV and existing fuel cell system parasitics, the algorithmestimates the minimum net power that the fuel cell system is expectingthe fuel cell power system to request. Rather than lowering the MAVunder all conditions, the present invention proposes to use a lowerinitial MAV under conditions with a high expected rate of damage andthen increase the MAV to a steady state maximum.

FIG. 1 is a simplified block diagram of a fuel cell system 10 includinga fuel cell stack 12, where the stack 12 includes a series of fuel cells30. A compressor 14 provides an airflow to the cathode side of the fuelcell stack 12 on a cathode input line 16 and a cathode exhaust gas isoutput from the stack 12 on a cathode exhaust gas line 18. The anodeside of the fuel cell stack 12 receives hydrogen gas from a hydrogensource 20 on an anode input line 20 and an anode exhaust gas is outputfrom the stack 12 on an anode exhaust gas line 24. A component 32, suchas a high voltage battery, is provided as a load for the power generatedby the stack 12 to maintain the cell voltage at or below the desiredmaximum set-point as discussed herein. A monitoring device 26 monitorsthe voltage of the cells 30 in the fuel cell stack 12 and a controller28 controls the operation of the system 10 including calculating theaverage cell voltage and generating the maximum voltage set-point of thecells 30, as will be discussed in detail below. The fuel cell system 10is intended to represent any fuel cell system suitable for the processdescribed herein, including anode recirculation systems, anodeflow-shifting systems, etc.

FIG. 2 is a graph with time on the horizontal axis, average cell voltageon the left vertical axis and minimum power provided to a fuel cellpower system on the right vertical axis. Graph line 40 shows an averagecell voltage over time and graph line 42 shows the minimum powerprovided to the fuel cell power system that will provide the averagecell voltage. The average cell voltage represented by graph line section44 is a low average cell voltage that may occur as a result of thesystem 10 being off, the system 10 being at high power or the operationof the system 10 being at a cathode stoichiometry approaching 1. Whenthe power request from the stack 12 goes low, and the average cellvoltage increases at point 46, the algorithm allows the average cellvoltage to quickly rise at graph line section 48 to a reduced averagecell voltage point 50, for example, 850 mV, that is below the voltagethreshold where significant dissolution of the platinum particles occursas a result of voltage cycling. Thus, limited cell degradation occurs,but limited platinum particle oxidation may begin to occur. In otherwords, the voltage at the point 50 is selected to be just below thevoltage where catalyst dissolution occurs, but where the voltage is highenough so that the stack 12 is not producing significant power that maybe used inefficiently. Generally, oxidation of the catalyst will beginto occur at the voltage point 50, and as the cell voltage increases, theoxidation coverage of the catalyst will also increase at some ratedepending on the particulars of the system, including the particularcatalyst, where the rate of increase may be linear or not.

From the point 50, the algorithm then causes the average cell voltage toslowly increase on graph line section 52 to a maximum target averagecell voltage at point 54. The target cell voltage at the point 54 is thesteady-state maximum average cell voltage that is desired at low systempower requests. The voltage at the point 54 is selected to be a desiredrelatively high voltage, for example, 900 mV, where catalyst degradationas a result of voltage cycling would occur, but the stack 12 would notbe generating significant power that may otherwise be usedinefficiently, as discussed above. The slow ramp up to the desiredtarget voltage gives the platinum particles time to oxidize before thecell voltage reaches the critical degradation voltage at the point 54.During the slow voltage ramp up on the section 52, more stack power mayneed to be dissipated than if the average cell voltage was allowed to goimmediately to the steady-state voltage, but the degradation of theplatinum particles is reduced because of the oxidation. When a powerup-transient is requested at point 56, the average cell voltage drops atgraph line section 58. The graph line 42 illustrates the minimum powerrequest from the stack 12 during the high average cell voltages.

Most of the time during normal system operation, the fuel cell stack 12is not at a sustained low voltage. If the stack 12 has been at highervoltages, a less aggressive initial MAV can be used. The easiestapproach to achieve this is to set the MAV as a function of cathodeplatinum oxide (PtOH) coverage. By knowing the platinum oxidation state,the voltage control of the system 10 can selectively determine thevarious voltage values that the maximum average cell voltage can be setto, and selectively determine the ramp up rate to the desiredsteady-state cell voltage for that oxidation level. An empirical dynamicPtOH model, that is a function of average cell voltage, can be developedfor this purpose, as would be well understood by those skilled in theart. A primary input to the PtOH model would be an IR(current-resistance) corrected average cell voltage, which is equal tothe average cell voltage plus stack current times cell resistance, wherethe cell resistance is either measured or estimated. Cathode relativehumidity and stack temperature could also be used as inputs with thecorrected voltage to generate a more accurate platinum oxidation state.The estimation of platinum oxide coverage could either be empiricallybased or more fundamental in form.

FIG. 3 is a flow chart diagram 60 for determining and using a minimumrequested power value from the fuel cell stack 12 that considers theoxidation state of the catalyst in the fuel cells 30, as discussedabove. The algorithm determines and/or retrieves the average cellvoltage, the stack current density and the internal resistance of themembranes of the cells 30 through various measurement and/or estimationprocesses at box 62 using any suitable technique, many of which would bewell understood by those skilled in the art. The algorithm uses thesevalues at box 64 to calculate the IR corrected average cell voltage ofthe fuel cells 30 in the stack 12 as referred to above. The algorithmthen uses the IR corrected average cell voltage at box 66 to determinethe oxidation state of the platinum or other catalyst. Other parameters,such as stack temperature, membrane humidification, etc., can also beused to determine the oxidation state of the platinum if desired. Thealgorithm then uses the oxidation state of the platinum at box 68 todetermine the maximum average cell voltage set-point of the fuel cells30 in the stack 12. The voltage set-point can be any voltage that allowsthe platinum particles to oxidize further, where the voltage set-pointmay be ramped up as discussed above depending on the oxidation state ofthe catalyst. That voltage set-point is then used to determine a minimumpower request from the fuel cell stack 12 at box 70 so that the powerdrawn from the stack 12 causes the fuel cells 30 to operate at thatvoltage set-point. The power generated by the stack 12, beyond what isrequired for normal vehicle operation, is then used at box 72 to chargethe battery or be dissipated in some component in the system 10depending on the power value and other factors.

The foregoing discussion disclosed and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A method for determining an average cell voltageset-point for fuel cells in a fuel cell stack, said method comprising:determining that the average cell voltage is at or near a first averagecell voltage level; allowing the average cell voltage of the fuel cellsto rise to a second average cell voltage level from the first averagecell voltage level in a first specified manner; determining an oxidationstate of a catalyst within the fuel cells; and causing the average cellvoltage to rise from the second voltage level to a third average cellvoltage level in a second specified manner that depends on the oxidationstate of the catalyst.
 2. The method according to claim 1 whereinallowing the average cell voltage of the fuel cells to quickly rise tothe second average cell voltage level includes allowing the average cellvoltage to the fuel cells to quickly rise in response to a low powerdemand on the fuel cell stack.
 3. The method according to claim 1wherein determining an oxidation state of a catalyst within the fuelcells includes determining an IR corrected voltage based on the averagecell voltage, stack current density (I) and cell membrane resistance(R), and using the IR corrected average cell voltage to determine theoxidation state of the catalyst.
 4. The method according to claim 3wherein determining the oxidation state of the catalyst includes usingstack temperature and membrane humidification in combination with the IRcorrected voltage to determine the oxidation state of the catalyst. 5.The method according to claim 1 wherein determining the oxidation stateof the catalyst includes using an empirical model.
 6. The methodaccording to claim 1 wherein the catalyst is platinum.
 7. The methodaccording to claim 1 wherein the second average cell voltage level isabout 850 mV and the third average cell voltage level is about 900 mV.8. A system for determining an average cell voltage set-point for fuelcells in a fuel cell stack, said system comprising: a controllerprogrammed to provide: means for determining that the average cellvoltage is at or near a first average cell voltage level; means forallowing the average cell voltage of the fuel cells to rise to a secondaverage cell voltage level from the first average cell voltage level ina first specified manner; means for determining an oxidation state of acatalyst within the fuel cells; and means for causing the average cellvoltage to rise from the second voltage level to a third average cellvoltage level in a second specified manner that depends on the oxidationstate of the catalyst.
 9. The system according to claim 8 wherein themeans for allowing the average cell voltage of the fuel cells to quicklyrise to the second average cell voltage level allows the average cellvoltage to the fuel cells to quickly rise in response to a low powerdemand on the fuel cell stack.
 10. The system according to claim 8wherein the means for determining an oxidation state of a catalystwithin the fuel cells determines an IR corrected voltage based on theaverage cell voltage, stack current density (I) and cell membraneresistance (R), and using the IR corrected average cell voltage todetermine the oxidation state of the catalyst.
 11. The system accordingto claim 10 wherein the means for determining the oxidation state of thecatalyst uses stack temperature and membrane humidification incombination with the IR corrected voltage to determine the oxidationstate of the catalyst.
 12. The system according to claim 8 wherein themeans for determining the oxidation state of the catalyst uses anempirical model.
 13. The system according to claim 8 wherein thecatalyst is platinum.
 14. The system according to claim 8 wherein thesecond average cell voltage level is about 850 mV and the third averagecell voltage level is about 900 mV.